High speed method of making cellulose organic derivative film and sheeting



y 1943. c, R. FORDYCE ETAL 2,319,054

HIGH SPEED METHOD O F MAKING CELLULOSE ORGANIC DERIVATIVE FILM AND SHEETING Filed Dec. 10, 1938 2 Sheets-Sheet 1 7 c o o o 16 3 o $5 4 4242 474% 5;, M5

CIMRLES R FORDYCZE MLKERRHUNTEKJR.

INVENTORS May 11, 1943.

HIGH SPEED ME'TH DERIVATIVE FILM led Dec.

c R. FORDYCEETAL OD'OF MAKING CELLUL 2,319,054 ORGANIC SHEEII 2 Sheets-Sheet 2 MLKEREHUNTERJZ.

TEMPERATURE c INVENTORS A IT0R%S CHARLES R FORDYCE Patented May 11,

HIGH SPEED METHOD OE MAKING CELLU- LOSE ORGANIC DERIVATIVE FILM. AND

SHEETING Charles R. Fordyce and Walker-I. Hunter, Jr.,

Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application December 10, 1938, Serial No. 245,o 22

.ZClaims.

This invention relates to a high speed method of making attenuated cellulose derivative products, such as film and sheeting, and more particularly to a method ofmakin such products which is characterized by the use of cellulose organic acid ester compositions of hitherto unknown properties. e

As iswell known, cellulose derivative sheets or films are ordinarily produced by depositing a cellulose derivative solution or dope in the form of a film on the highly polished surface of a slowly rotating wheel or band, causing the film to set by evaporation of solvent, stripping the film and curing out residual solvent. The dope compositions heretofore. employed for this purpose have been solutions which se or reach a solid or semi-solid condition, permitting removal from the forming surface only by gradual evaporation of solvent. 'With such dopes most of the solvent must be removed (leaving not much more than 20-25% of solvent, based upon the weight of the sheet) before satisfactory stripping of the film can be accomplished.- This necessitates a relatively long period of preliminary curing on the wheel. Furthermore, the length of time required for proper setting is increased by the fact that, since such dopes remain fluid or semi-fluid until most of the solvent has evaporated (and, therefore, must be supported on the wheel surface), evaporation of solvent can take place only from the outside surface of the deposited film. In addition, such be brought, immediately after castin8.- into a set or non-fluid condition while still containing all, or nearly all, of its original solvent-a condition which would permit almost immediate stripping (thus reducing strippin time to a minimum) and curing solventfrom both surfaces of the film simultaneously.

, Numerous attempts have been made to realize this ideal. For example, it has been proposed to use mixtures of lowand high-boiling solvents in the dope, so chosen that by rapidly evaporating the low boiling component a very concentrated solution of the cellulose derivative in the high boiling component would remain. It has alsobeen proposed to coagulate cellulose derivative solutions by means of non-solvent liquids or vapors. While such expedients have resulted in some improvement, until the advent of the present invention the ideal operation has never been attained.

As a further indication of the state of the art.

it may be said that the broad phenomenon of gelatin of certain types of cellulose derivativesolutions under' the influence of temperature change has been observed from time to time by various workers in the cellulosic field. It

' has been recognized, for example, that certain dopes tend to skin over on the outside surface because of more rapid loss of solvent from the upper layers of film material and this further increases the setting time.

The advantages of bringing the film material into a solid or semi-solid condition as early in the film-forming operation as possible are ap parent. Obviously, any reduction in the stripping time, that is, the timeduring which the film must remain on the wheel before it can be properly stripped, directly increases production speed. Moreover, if the film can be removed from the wheel while still containing considerable solvent, more rapid curing can be attained, because under such conditions thefilm can be so handled and treated as to permit curing out of, solvent from both surfaces simultaneously. An additional advanta e is that early solidification or colloidization results in a preferred micellar mat-like structure with attendant improvement in the quality of the finished-product. The ideal film-forming operation would, therefore, be one in which the dope could organic liquids which arenon-solvents for cellulose acetate and other cellulose organic acid esters at ordinary temperatures become solvents at elevated or moderately elevated temperatures and that if solutions are formed at the high temperatures and coated on a metal or other surface and cooled down, a tenaciously adhering lacquer coating results. It has also been recognized that by heating a suspension of cellulose acetate in ethylene dichloride (a cellulose acetate non-solvent at ordinary temperatures) to about 30-60 C., the cellulose acetate goes into solution to form a clear solution and when such a solution is coated on a surface, cooled and cured to remove the solvent, 9. clear transparent film results. In other words, while a hot ethylene chloride solution of cellulose acetate will gel up'on coating or casting upon a film-forming surface, this phenomenon does not increase the speed of production of sheeting therefrom because such a film cannot be stripped and handled while containing any more solvent than the ordinary cellulose acetate dope wherein acetone and the like are solvents. In other words the gel so formed is not self-supporting. workers in this field have never gone much beyond a recognition of the phenomenon that certain dopes are capable of gelling and others are not Until the present invention, no practical application of the phenomenon of gelation to filmforming operations has ever been made.

This invention has as an object to provide a high speed method of making cellulose derivative sheeting adapted for use as photographic film support and for other purposes. A further object is to provide a method of making cellulose organic acid ester film or sheeting by coating or casting a dope on a film-forming surface-characterized by the fact that the film may be removed or stripped from the surface while still containing a large proportion of solvent. A still further obiect is to provide a method of making such film or sheeting in which film formation takes place almost immediately upon deposition of the dope. Another object is to provide a method of cellulose ester film or sheet formation in which the film can be removed from the forming or casting surface almost immediately after gelation while containing large proportions of solvent and is in such condition that residual solvent may readily be cured out of both surfaces simultaneously. An other object is to produce cellulose organic dcrivative sheeting having high tensile strength and flexibility and a low swell and shrink amplitude. Other objects will appear hereinafter.

These objects are accomplished by the follow- J ing invention, which, in its broader aspects, comprises dissolving at elevated temperatures or moderately elevated temperatures certain cellulose acetate propionates and cellulose acetate butyrates in a solvent consisting of a mixture of trichloroethylene and monohydric aliphatic alcohols of not over five carbon atoms, whereby a solution or dope is obtained which is susceptible of gelation by rapid lowering of temperature to produce a sheet or film having such strength in the gel state that it may be stripped from the casting surface almost immediately after casting and while still containing nearly all or at least a large proportion of the original hot solvent.

We have found that solutions of this character, the composition and preparation of which will be described in more detail hereinafter, possess certain unusual and unexpected characteristics which render them outstanding for the specific purposes of the instant invention. Among other things, (1) they are fluid at temperatures above 50 0.; (2) when allowed to cool to or below a critical temperature between -50 C. (depending upon the composition) they form entirely transparent gels which remain homogeneous throughout the gelling operation, such gelation occurring within approximately 20 C. of the flowable solution point; (3) the gels when first formed do not adhere strongly to surfaces such as metal, glass, etc.; (4) the gels are sufiiciently strong and resistant to deformation that they can be handled while still containing large quantitles of solvent, 1. e., an amount of solvent equal takesint'o account the fact that ordinary filmtorming processes generally involve the use of dopes which require in some cases as much as fifteen or twenty minutes preliminary curing on the casting wheel or other surfaces before the material reaches a stage in which it can be successfully stripped, the tremendous increase in manufacturing speed made possible by the present method will be apparent.

In the following examples and description we have set forth several of the preferred embodiments of our invention, but they are included merely for purposes of illustration and not as a limitation thereof. I

In the accompanying drawings:

Fig. 1 is a diagrammatic elevational sectional view of a conventional type of device which may be employed for carrying out a typical film-forming operation in accordance with our invention.

Fig. 2 is a chart showing graphically the various cellulose organic acid esters which may be employed within the teaching of our invention.

Fig. 3 is a graphical representation of the viscosity changes which occur when certain typical compositions of our invention are cooled from their solution temperatures to or below their gelation temperatures or temperature ranges.

Referring first to Fig. 2 it is a triangular chart to identify the chemical composition of the cellulose esters under consideration. Ihe composition in per cent acetyl is plotted along the line AB! and the per cent higher acyl (such as propionyl or butyryl) is plotted along the line AC. The points ta, tp and "tb represent cellulose triacetate, tripropionate and tributyrate, respectively. The line connecting ta and tb represents fully esterified mixed esters of acetic and butyric acid and the line connecting ta. and "tp" represents fully esterified mixed esters of acetic and propionic acids. Hydrolyzed mixed esters fall within the areas lying above the fully esterified products.

On this chart are outlined shaded areas I and II representing the cellulose organic acid esters susceptible of forming solutions of the gelation type in mixtures of trichloroethylene and various monohydric alcohols of not over five carbon atoms, in accordance with our invention.

In order to prepare a solution of a cellulose mixed ester within this area which will exhibit the property of forming a gel upon allowing the warm solution to cool it is desirable to employ the optimum proportions of trichloroethylene and a suitable alcohol for the particular composition of cellulose ester to be used. In the regions of lower propionyl or butyryl content as outlined in area I of Fig. 2 it would be desirable to use comparatively low boiling alcohols such as those of not more than three-carbon atoms or the tertiary alcohols of four and five carbon atoms. 'Ethyi and methyl alcohol should be used in quantities of 10-30% of the solvent mixture and normalor iso-propyl alcohol, tertiary butyl and tertiary amyl alcohols in quantities within the range of 20-50% by weight of the solvent mixture, the balance being trichloroethylene. With cellulose esters of greater higher acyl content, as outlined in area II of Fig. 2 one could employ alcohols such as normalor iso-propyl, tertiary butyl or tertiary'amyl in quantities of 40-60% by weight of the solvent mixture or any of the various butyl or amyl alcohols within the range of 30-60% by weight of the solvent mixture.

' Our invention will be better understood from the following tabulation offhot solvents which may be employed with the cellulose mixed ore ganic acid esters designated by the areas I and II of the chart of Fig. 2.

herein and m the claims to cellulose acetate propionates or cellulose acetate butyrates containing Percentages of alcohols in trichloroethylenealcohol solvent mixture Area I (esters of -25% higher acyl) Area II (esters of to approx. 40% higher acyl) A B C I) Methyl 10-307 -n-Propyl M7 n-Prop 40-607 n-Butyl -609 Ethyl 1 Iso-prop 20-5072 Iso-propyl 40-60% Iso-butyl... 30-60%: tert. Butyl 20-50% tert. Butyl 40-60 0 sec. ButyL. 30-60% tert. Amyl 20-50% tcrt. Butylm 40-60 0 n-Amyl 30-60% Iso-amyl 30-00% sea-Amy] 30-60% As a guide to the more exact proportions of the various aliphatic alcohols employed with the various cellulose mixed organic acid esters, it may be stated that the higher the percent of acyl, or the higher the molecular weight of the acyl radical, the more soluble the ester is; also the more soluble the ester or the more active the solvent the lower will be the gelling temperature and, in fact, if too active a solvent is employed, the gelling phenomenon will not operate to give a self-supporting gel.

Thus, solvent mixtures containing methyl and ethyl alcohol, if used with cellulose esters within area'lI of Fig. 2 would tend to give solutions at room temperatures rather than the desired gels and alcohols listed under group C in the above table would tend to give solutions at room temperatures if employed in quantities less than 40% with cellulose esters within area 11. The gelation temperatures of solutions within any of the solvent compositions as outlined under group C or D will tend to be higher with quantities of alcohol nearer the upper limits of the specified ranges of composition. Thus a cellulose acetate butyrate of 26% acetyl and 22% butyryl content when dissolved in a solvent mixture containing 20% iso-propyl alcohol exhibits a gelation temperature of C. while the same cellulose ester dissolved in a solvent mixture containing 30% iso-propyl alcohol, the balance of solvent in each case being trichloroethylene exhibits a gelation temperature of C. In a similar way showing the efiect of increased amount of higher acyl content on the gelation temperature, a cellulose acetate butyrate of 33% butyryl content in a solvent mixture composed of 50% iso-propyl alcohol and 50% trichloroethylene exhibits a gelation temperature of 35 C. while a cellulose acetate butyrate of 36% butyryl content in the same solvent mixture exhibits a gelation temperature of 30 C.

It will be understood from the chart ofvFig. 2 and the above table that the cellulose esters employed in accordance with our invention are cel- .lulose acetate propionates and cellulose acetate butyrates containing not less than about 15% about 42%, or more specifically, those cellulose esters having a composition falling within areas I and II of Fig. 2. It should be noted however that the numerical ranges of acyl content just given are not exact, except as referred to the higher acyl and not over about 40% total acyl, and having a total acyl content of not less than not more than about 40% higher acyl, we refer specifically to those esters which fall to the left of the curved line forming the right-hand boundaries of area II of Fig. 2.

Plasticizers may be used in varying quantities in the above compositions and have a minor effect upon the gelation behavior. Use of triphenyl phosphate in quantities as high as 25% of the weight of the cellulose ester does not produce any measurable change in gelatlon temperature, stripping time, or other phases of the filmforming operation. Liquid plasticizers used in large quantities usually require a'minor adjustment in solvent mixtures, such as a decrease in the quantity of more active solvent by 5-10%.

We have referred to the viscosity characteristics of the various compositions adapted for use in our process, audit is accordingly desirable at this point to describe the method by which viscosity is measured. This is a modification of the widely used dropping ball method," the procedure being as follows:

The dope under examination isfiltered and poured into a test tube having a depth of mm., a diameter of 15 mm. and containing a steel ball 1%" in diameter weighing .4400 gram The tube is filled to the brim with the dope under test and a cork stopper inserted with pressure enough to force air bubbles andexcess dope past the cork. A small wire may be placed along-side the cork to facilitate the passage of air bubbles and dope past the stopper. The glass tube carries two scratches positioned exactly 10 cm. apart. The dope-filled tube is then placed vertically in a constant temperature water bath with the stopper down. After the bath and tube have reached equilibrium temperature (usually within a peribd of one-half to one hour), the tube is quickly inverted and placed in a vertical glass cylinder placed in the water bath. When the bottom of the steel ball reaches a position level with the first scratch, a stop watch is started and the time required for the bottom of the steel ball to reach a position level with the se ond scratch is measured. The viscosity is recorded as the time in seconds required for the ball to travel this 10 cm. distance between the two scratches.

The viscosities referred to in the specification 4 and in the claims are to be understood as having designates adope storage or supply tank provided with an inlet conduit 2 for admission of the previously prepared dope. The tank is provided with a removable cover 3 for permitting inspection of the contents and for other purposes and also provided with a heating coil 4 through which a flow of an appropriate heating fluid such as hot water or steam is maintained by means of thermostatically controlled valve 5. The flow of the heated fluid is so regulated as to maintain the dope in the tank i at a constant temperature.

Numeral 6 designates a feed conduit (which may be provided with lagging of an appropriate type for preventing heat losses as far as possible) through which the heated dope is passed to a standard form of dope hopper 1, fiow of the dope being controlled by means of valve 8.

The dope hopper is provided with an adjustable gate member 9 for controlling the thickness of the dope stream which flows from the hopper. Adjustment of the gate member 9 may be by thumb screw I threaded through one wall of the hopper. The hopper is provided with a cover It to prevent solvent and heat losses and is also preferably supplied with external or internal heating means (not shown) for maintaining the dope at a constant temperature.

Positioned below the hopper I is the coating or casting wheel l2 mounted in suitable bearings i3 and surrounded by air casing ll, the wheel being adapted to rotate in the direction indicated by the arrow. The wheel is provided with appropriate cooling means (not shown) whereby its film-forming surface is cooled to an appropriate temperature equal to or below the gelation temperature of the particular dope employed in a given film-forming operation. Casing I4 is provided with air inlet conduit l5 and outlet conduit It for conducting a current of heated air around the wheel counter-currently to the path of the film undergoing formation. 1

The wheel is driven by appropriate mechanism (not shown) of such nature that any desired rotational speeds may be attained. Numeral ll designates a conventional stripping roll over which the formed film passes on its way to the curing device, which comprises a plurality of air sections l8, l9, and 20. These air sections are provided, respectively, with air inlet conduits 2|, 23, and and with air outlets 22, 24, and 28 which provide a means of conducting heated air through each section in the general direction indicated by the arrows.

Numeral 21 designates a guide roll over which the film passes, after leaving stripping roll II, on its way to the first air section. Numerals 28, 29, 3., etc., designate a series of rolls in the respective air sections over which the film or sheet material passes on its way to the wind-up II located in the last air section 2.. These rolls are driven, preferably by means of the so-called tendency drive which permits the film to travel through the air section in a substantially freely supported condition, this type of drive compeneating for any longitudinal changes of dimension which may take place in the film material during the curing operation.

The numeral 58 designates a hinged door which gives access to the last air section 2. and through which rolls of the finished product may be removed from time to time.

A typical film-forming operation may be carried out as follows:

wheel surface, at a temperature well above its gelation point and in a readily fiowable condition.

The warm dope passes by means of conduit 6 into dope hopper 'I from which it flows onto the wheel in a stream, the thickness of which is regulated by appropriate adjustment of gate member 9 to give the desired eventual film thickness, for example, .005 inch.

As previously indicated, the wheel surface is maintained at a temperature equal to or below the gelation temperature or temperature range of the particular dope in question and the wheel is driven at such a peripheral speed as to give the desired speed of film formation. As the dope contacts the cold wheel surface gelation takes place almost immediately, and, at the expiration of a substantially insignificant period of time, the film material has reached a condition in which it may be removed from the wheel at the stripping roll ll. Although it is not necessary to subject the film to any considerable amount of curing on the wheel, it is generally best to remove a certain amount of solvent from the gelled film material at this point in th process. To this end air is admitted to wheel casing ll through conduit l5 and passes countercurrently around the outside surface of the film, the solvent-laden air being finally conveyed out of the apparatus through conduit Ii. The air temperature may be adjusted to or below room temperature or it may be heated to as high as approximately 40 C. or over, the particular temperature depending upon the composition of the dope in question, the wheel speed, and various other factors.

The nature of the dop being such that it sets almost immediately into a rigid gel upon contacting the cold wheel surface, the film may be readily stripped upon reaching stripping roll l'l. At this point th film contains a substantial amount of solvent, the exact amount, of course. being dependent on wheel speed, temperature of the casing air and other factors. As will be apparent, when it is practical to operate the wheel An appropriate dope composition, previously thoroughly mixed in another container at an appropriate temperature, is fed into the mixing tank i through the conduit 2. Care is taken to maintain the dope, prior to contact with the at a sufiiciently high speed, the film may be removed from the film-forming surface while still containing practically all of its original solvent.

Under no circumstances is it necessary to bring the solvent content down to a point below that at which the weight of the solvent equals the weight of the cellulose organic acid ester. Under ordinary circumstances the wheel is operated at such a speed that the film contains anywhere from 50% to of solvent at the time of strip- Pint.

After stripping, the film is conducted into the first air section I, where it is subjected to the action of a current of air heated, for example, to about 40-60 C. Solvent is removed progressively with travel of the film through the air section. The film upon emerging from the first air section passes immediately into the next air section where it is subjected to the action of air heated to a temperature of about 40-80 C. and finally into the air section 20, where it is subjected to the action of air heated from about -95 C. By

the time the film reaches the wind-up 55 it has lost substantially all of its original solvent content and is then in suitable condition for use as photographic film support and many other purposes.

. invention will be more readily understood by reference to a number of specific examples illustrating preferred embodiments thereof.

Example 1.A solution of parts by weight of a cellulose acetate butyrate 0f 28% acetyl and 16% butyryl content in 500 parts by weight of a solvent mixture composed of 70% trichloroethylone and 30% isb-propyl alcohol was prepared at 60 C. The solution was coated in a thin layer of uniform thickness onto a highly polished filmforming surface having a temperature of about 20 C. The solution set almost immediately to a rigid gel under the influence of the lower temperature. The material was allowed to stand in a current of air at approximately 20 .C. for two minutes whereupon it was stripped from the surface and cured to remove volatile solvent. The resulting clear, transparent film was .005 inch in thickness and was found by test to have good tensile strength and flexibility.

Example 2.A solution of 100 parts by weight of a cellulose acetate propionate containing 28% acetyl and 17% propionyl content in 500 parts by weight of a solvent mixture composed of 70% by weight of trichloroethylene and 30% iso-propyl alcohol and containing triphenyl phosphate based on the weight of the cellulose ester, was prepared by mixing the ingredients with continued stirring at 60 C. The solution was then filtered to remove incompletely dissolved particles film thickness of .005 inch, the wheel being maintained at a constant temperature of about 20 C.

The wheel was rotated at a speed such that the film remained on the film-forming surface for about three minutes during which time a current conditions of coating. The finished film was found to have high tensile strength. 111811 flexibility, and a low swell and shrink amplitude.

Example 3.-A solution prepared from 100 parts by weight of a cellulose acetate propionate of 28% acetyl and 17% propionyl content in 500 parts by weight of a solvent mixture composed of 80% trichloroethylene and 20% methyl alcohol wasprepared at 60 C. and the resulting solution allowed to cool to approximately 40 C. The solution at this temperature was allowed to fiow onto a casting surface at 20 C. in a thickness such that the cured film was approximately .005 inch thick, the freshly coated :lilm while still containing a quantity of volatile solvent equal to more than the weight of the cellulose ester was removed from the coating surface and the solvents removed by a current of warm air. The resulting clear transparent film exhibited markedly Physical properties superior to those obtained from a similar cellulose ester coated from known solvent mixture such as acetone.

With a procedure similar to that in theprevious examples, solutions .were prepared from a variety of cellulose acetate propionates and cellulose acetate butyrates'in solvent mixtures of trichloroethylene and suitable alcohols. When the warm solutions were allowed to flow on a filmforming surface at 20 0. rapid solidification of the films occurred,-enabling them to be removed from the casting surface within very short periods of time. The compositions of the cellulose esters and solvent mixtures, minimum stripping time.

35 and gelation temperature in these experiments Total 4 4 Example Per cent higher acyl solvent Alcohol 5%? 16 307 b tyryl 500 I00 not ito'y ifo as u 16 l l mA l o 15.4 15 1o prop ony 1.0 3s 15 110% *gropionyl.-. 00 1.4 35 21 utyrl m 0.5 20 25 propionyl..- 400 1.0 30 28 17 propionyl 500 0. o 40 of air having an inlet temperature of about -C. was passed through the space around the wheel in a direction counter-current to that of the movement of the film.

The warm dope, immediately upon comingin contact with the cold wheel surface, was transformed into a non-fluid gel. After completing somewhat more than three-quarters of a revolution on the wheel, the film was stripped from the film-forming surface and was thereafter carried through the three air sections where it was subjected to the curing action of a current of moderately heated air. The air passing through the first air section had an inlet temperature of about 50 0. providing an average temperature in the section of 45 C. The path and speed are such that the film in this section took approxi-, mately 16 minutes to travel therethrough. The

average temperature-of the second air section was 65 0., and of the third section 80 0., the path and speed of the film being such that any, given portion thereof remained in these air sections for a period of approximately 16 minutes.

It will of course ,be understood that the particular film-forming composition employed in a given case will depend upon such practical matters as the temperature of the casting surface which it is desired or practical to employ, the temperature at which the cellulose ester may be placed in solution and various other factors. For example. as the proportion of alcohol in the solvent mixture is varied for a given cellulose ester, the temperature of the casting surface required to produce gelation will also be varied as above indicated. As previously pointed out, optimum compositions for practical .use are usually obtained by use of a quantity of alcohol near the upper limit of concentration for a given cellulose ester, when compositionsof groups C and D of the table on page 3 are employed, while for compositions tabulated under A and B proportions of alcohol near the center of the range arepreferred.

In accordance with this general principle we have found the compositions of Examples 1, 3, and a to be outstanding in their ability to give satisfactory photographic film base ,when em-,

ployed in accordance with our invention.

the matter of permissible solvent content at stripping is one of the distinguishing features of our invention. Film or sheet material produced according to the above-mentioned prior art method must be cured on the film-forming surface until residual solvent is reduced to or below about 10-20% before satisfactory stripping can be attained. Our compositions, on the other hand, are of such nature that they may be satisfactorily stripped from the film-forming surface while containing anywhere from 50 to 80% solvent. It will thus be seen that the film or sheet material of the instant invention is of a fundamentally different nature than similar products produced from the nonlling types of dope of the prior art.

One of the most outstanding differences between films or sheets produced in accordance with our invention, and similar prior art products, is the fact that they have an extremely low swell and shrink amplitude," that is, the property of undergoing linear dimensional change in alternately wet and dry condition. As is well known, the swell and shrink characteristics of a photographic film, for example, are of great importanceand the most useful films are those having the lowest swell and shrink amplitude.

This is of particular importance in films which are to be used for X-ray, portrait, or aerial photography where sheets of appreciable size are employed. Obviously films of high swell and shrink characteristics tend toward internal unevenness which is due, either to buckling of the film in the center, or to curling of the edgesphenomena which are absent from films having a low swell and shrink amplitude and the ability to lie fiat without curling. Other types of film which are used in long strips, such as rolls of Cine films, are difilcult to process-such materials if of high swell and shrink characteristics, exhibiting appreciable shrinkage after removal from developing or washing solutions, at which time the films are usually mounted on a drying rack. Under such conditions these films tend to become severely tightened resulting in distortion of the film base and the photographic image carried thereby.

- The natures of our film-forming compositions will be more readily understood by reference to Fig. 3 of the drawings which illustrated graphically the change in viscosity which our solutions undergo upon lowering the temperature. Curve A was plotted from viscosity determinations made at various temperatures upon a film-forming solution composed of 100 parts of a cellulose acetate butyrate of 28% acetyl and 16% butyryl content in 500 parts of a solvent mixture of 70% trichloroethylene and 30% iso-propyl alcohol. It will be noted that the curve rises gradually as the solution is cooled from 70 C. and that upon approaching the temperature range of 30 to 45 0., a very marked increasedn viscosity takes place. Continued cooling below about 25 C. results in extreme viscosity and gelation with the production of a, rigid non-fiuid mass. By employing varying concentrations 01' the cellulose ester in solution, the character of the curve is'found to change somewhat. A more concentrated solution of the same cellulose ester in the same solvent combination would give a curve of the type "B," while a lower concentration of the cellulose ester would give curve C.

example, one may increase or decrease the temperature at which gelation will occur for a given solvent combination by varying the proportion of trichloroethylene of the solvent composition.

It will be seen from the above examples that no hard and fast rules can be laid down as to the composition of our film-forming solutions for all purposes, since the composition of a given solution will be adjusted in accordance with the particular conditions of coating, stripping and curing which are to be employed. In general, it may be said that for a practical process a given composition should be, in accordance with our invention, such that the cellulose derivative in question goes into solution at temperatures at or above 50 C. and remains fluid above that temperature. It should also be such that upon cooling it experiences a rather sharp increase in viscosity within a comparatively narrow temperature range of about 20 C.

It will be apparent that in a practical filmmaking operation many variations in the solution temperature, wheel temperature, wheel casing air temperature, curing temperature, wheel speed, and many other details of the process may be made within the scope of our invention. As previously indicated, when employing compositions which are solutions above 50 C., the wheel temperature may be in the neighborhood of 10 to 20 C., or at least sufiiclently low to bring the dope to, and preferably below its gelation temperature.

At this point it may be well to discuss gelation temperature. By this term we do not necessarily refer to an exact temperature, but rather to a maximum temperature below which the cooling solution or dope undergoes a marked and rather sudden increase in viscosity. While no exact maximum can be specified which will cover all possible cases, we may say that gelation or solidification of those compositions which we,

tem-

- or film material produced in accordance with our invention may be carried out as set forth above by standard curing procedures, that is, b conducting the material through appropriate curing chambers where it is subjected to the action of air maintained at elevated or moderately elevated temperatures. It is desirable to subject the film material to low tension during the curing operation in order that the final product may have the desired physical properties. In fact, the

Our process has many advantages over known film-making processes, but the most outstanding advantage is the tremendous increase in speed of film formation obtainable thereby. While we have referred to stripping times of anywhere from a minute or two to five or six minutes, there is no actual theoretical limit to the stripping time, short of zero. In other words, according to our process, film or sheeting may be stripped almost immediately after coating. It will be appreciated, however, that the actual speed of a given practical film-making operation will be considerably lower than that theoretically obtainable. The operation may be slowed down by the practical necessity or desirability of applying various subbing or backing treatments to the fllmsupport during the manufacture operation. As a general proposition, it maybe stated that the film-making speeds obtainable by our process are far beyond anything which has thus far been obtained in the film-making industry. For example, anywhere from ten to twenty minutes are required to cast and strip a film under published procedure, whereas film may be cast and stripped by our process within a minute or even less from the time of deposition of the film-forming composition.

One of the distinguishing and unusual features of our invention is the fact that, due to their peculiar composition and characteristics, satisfactory gelling of our film-forming compositions is quite independent of the thickness of the deposited layer, although the thicker the layer, the lower is the casting speed due to the relatively lower heat transference of thick layers as compared to thin layers. We may, however, produce films or sheets anywhere from a few ten thousandths inch or less to almost any desired thickness. It will thus be seen that our process is adapted, not only for the manufacture of photographic film support and even much thinner types of sheeting, such as those employed for wrapping purposes, but also for the manufacture of sheets adapted for use in the fabrication of laminated glass, container stock, and many other products.

What we claim is:

1. A high speed gelation process' of making sheeting suitable for photographic film base which comprises dissolving at a temperature above 50 C. a cellulose mixed organic acid ester selected from the group consisting of cellulose acetate propionates and cellulose acetate butyrates containing not less than about 15% and not greater than about 40% higher acyl and containing not less than about 42% total acyl, said esters having the composition indicated by areas I and II of Fig. 2 of the drawings, in a liquid which is a solvent for the said cellulose ester only at a temperature above 50 0., and in a weight of such liquid greater than the weight of the cellulose ester dissolved which will give a solution which at a temperature within the range of -50 0., will form a clear, transparent. selfsupporting gel and which liquid is composed of a mixture oi trichloroethylene and a monohydrlc aliphatic alcohol of not more than five carbon atoms, said liquid being selected from the group consisting of mixtures composed, respectively, of 90-70% trichloroethylene and 10-30% methyl alcohol, 90-70 trichloroethylene and 10-30% ethyl alcohol, 80-40% trichloroethylene and 20-60% npropyl alcohol, 80-40% trichloroethylene and 20- 60% iso-propyl alcohol, 80-40% trichloroethylene and 20-60% tertiary butyl alcohol, 80-40% trichloroethylene and 20-60% tertiary amyl alcohol, and IO-40% trichloroethylene and 30-60% nbutyl alcohol, -40% trichloroethylene and 30- 60% iso-butyl alcohol, 70-40% trichloroethylene and 30-60% sec. butyl alcohol and 70-40% trichlorcethylene and 30-60% n-amyl alcohol, 70- 40% trichloroethylene and 30-60% 'iso-amyl alcohol and 70-40% trichloroethylene and 30-60%' sec. anryl alcohol, casting the solution from a supply thereof having a temperature above its gelation temperature in the form of a film at a temperature of 10-50 C. on a film-forming surface, stripping the film from the film-forming surface while containing at least 50% solvent and removing residual solvent from the film.

2. A gelable composition comprising a cellulose mixed organic ester selected from the group consisting of cellulose acetate propionate and cellulose acetate butyrates containing not less than about 15% and not greater than about 40% higher acyl and containing not less than about 42% total acyl, said esters having the composition indicated by areas I and II of Fig. 2 or the drawings dissolved in a liquid which is a solvent for the said cellulose ester only at a temperature above 50 C., said liquid being composed of a mixture of trichloroethylene and a monohydric aliphatic alcohol of not more than five carbon atoms, said liquid being selected from the group consisting of mixtures composed, respectively, of 90-70% trichloroethylene and 10-30% methyl alcohol, 90-70% trichloroethylene and 10-30% ethyl alcohol, -40% trichloroethylene and 20- 60% n-propyl alcohol, 80-40% trichloroethylene and 20-60% iso-propyl alcohol, 80-40% trichloroethylene and 20-60% tertiary butyl alcohol, 80- 40% trichloroethylene and 20-60% tertiary amyl alcohol, and 70-40% trichloroethylene and 30-60 n-butyl alcohol, 70-40% trichloroethylene and 30-60% iso-butyl alcohol, 70-40% trichloroethylene and 30-60% sec. butyl alcohol and 10-40% trichloroethylene and 30-60% n-amyl alcohol. 70-40% trichloroethylene and 30-60% iso-amyl alcohol and 70-40% trichloroethylene and 30-60% sec. amyl alcohol, and said liquid being of a weight, greater than the weight of the cellulose ester dissolved, which will give a solution which will form a clear, transparent, self-supporting gel at a temperature within the range of 10-50 C. which at that temperature is sufllciently strong and resistant to defamation to permit handling while containing more than 50% solvent.

CHARLES R. FORDYCE. WALKER F. HUNTER, JR.

CERTIFICATE OF conmscmmn. Patent No. 2,319,05h. May 11, 1915.

CHARLES R. FORDYCE; ET AL.

It is hereby certified that error appears 1n the printed specification of the above numbered patent requiring correction as follows: Page 7, secand column, line 27, claim 2, after the word "organic" insert "acid"; and that the said Letters Pateht should be read with this correction therein that the same may confozm to the record of the case in the Patent Office.

Signed and sealed this 27th day of. July, A. D. 19L

' Henry Ven Arsda1e, (Seal) Acting commissioner of Patents. 

